In an optical fiber communication system, it has been considered to use a multilevel modulation technique and a polarization multiplexing technique in order to increase the transmission rate per one wavelength from 10 Gbps to 40 Gbps or 100 Gbps. The multilevel modulation technique is a technique that uses an optical signal amplitude together with phase information to transmit more many information in one symbol. With regard to 40 Gbps transmission, Differential Quadrature Phase-Shift Keying (DQPSK) modulation has been already used according to which four-state signal can be used to transmit 2-bit information per one symbol. The polarization multiplexing technique is a technique to use polarization to multiplex two types of signals to thereby obtain a doubled transmission rate. In the 100 Gbps-class transmission which is expected to be put to practical use in the future, the polarization multiplexing QPSK modulation method, which is a combination of the above multilevel modulation technique and this polarization multiplexing method, has been considered as one of promising candidates.
These modulation methods are based on a modulator for generating a QPSK modulation signal. This modulator is different from a modulator consisting of a single Mach-Zehnder modulator conventionally used for 10 Gbps transmission for example in that, as shown in FIG. 1A, each arm waveguide section of a large MZI (hereinafter referred to as “parent MZI”) has an MZI modulator (hereinafter referred to as “child MZI”) to provide a modulator having a slightly-complicated configuration in which the MZIs are provided in a nested structure (hereinafter referred to as “nest MZI modulator”) (see Non-patent Publication 1, FIG. 10 for example). Generally, these modulators are prepared by a technique using lithium niobate light waveguide (hereinafter referred to as “LN waveguide”) having a high electro-optic effect (EO effect). Each child MZI includes a high-frequency electrode to convert an electric signal to an optical modulation signal (the above high-frequency electrode generally has a traveling wave electrode configuration and a transmission line structure having a fixed impedance is used for the electrode configuration. However, this drawing does not show a signal input section or a signal output section for the purpose of simplicity. The same also applies to the following drawings in principle). Each child MZI has a modulation operating point adjusting section having a bias electrode for adjusting a modulation operating point (which is connected to bias terminals BiasI and BiasQ, respectively. Hereinafter, a bias terminal will be referred to as a bias electrode for the convenience of the description). A parent MZI has a relative phase adjusting section that has a bias electrode Bias90° for adjusting relative phase of the optical signals outputted from the child MZIs (90° phase adjustment). This drawing shows an example in which an X-cut substrate is used that can reduce the number of high-frequency electrodes. The nest MZI modulator using the LN waveguide as described above (monolithic-type nest MZI modulator) is already commercially available and can be obtained widely.
In order to understand the QPSK modulation operation by the nest MZI modulator, the following section will firstly describe the operation of a child MZI (i.e., a single MZI modulator). An X-cut substrate is configured so that the LN polarization direction is in the up-and-down direction in FIG. 1A. The electric field from the high-frequency electrode at the center of the MZI modulator proceeds through an upper arm in the upper direction and proceeds through a lower arm in the lower direction. Thus, this electric field proceeds in an opposite direction to the polarization direction depending on the upper or lower arm waveguide (see FIG. 1B and FIG. 1C). Thus, a refractive index change due to the EO effect is in an opposite direction depending on the upper or lower arm waveguide. Thus, the phase change of light propagating through the waveguide is also in an opposite direction depending on the upper or lower arm waveguide. In the drawings, the electric field is shown so that only the electric field distribution applied to the waveguide is simply shown for the convenience of the drawing. Thus, the electric field distributions of other regions are not shown. This applies to the subsequent drawings.
The continuous wave (CW) light inputted to the MZI modulator is bifurcated by an optical coupler and is subsequently subjected, by the electric signal applied to the high-frequency electrode, to phase changes by the upper and lower arm waveguides that are in opposite directions respectively and that are in the same amount. Then, the bifurcated lights join together through the optical coupler again. Then, the electric field phase of the output signal light changes as shown in FIG. 2A. The light having passed the upper arm is subjected to a phase change in a positive direction. Thus, the electric field vector EMZI(H) draws a trajectory in a counterclockwise direction (X→white circle→black circle). The light having passed the lower arm is subjected to a phase change in a negative direction. Thus, the electric field vector EMZI(L) draws a trajectory in the clockwise direction. The vector synthesis of these electric fields results in the electric field vector EMZI of the output signal light. Thus, the trajectory draws a straight trajectory on the real axis. This can be described by the following formula.
                                          E                          MZI              ⁡                              (                H                )                                              =                                    1              2                        ⁢                          ⅇ              jξ                                      ⁢                                  ⁢                              E                          MZI              ⁡                              (                L                )                                              =                                    1              2                        ⁢                          ⅇ                              -                jξ                                                    ⁢                                  ⁢                              E            MZI                    =                                                    E                                  MZI                  ⁡                                      (                    H                    )                                                              +                              E                                  MZI                  ⁡                                      (                    L                    )                                                                        =                                                            1                  2                                ⁢                                  (                                                            ⅇ                      jξ                                        +                                          ⅇ                                              -                        jξ                                                                              )                                            =                              cos                ⁡                                  (                  ξ                  )                                                                                        Formula        ⁢                                  ⁢        1            
In the formula ξ represents a phase change applied by the electric field from the high-frequency electrode. Thus, when the MZI modulator of the X-cut substrate is driven (2Vπ-driven) as shown in FIG. 2B so that the phase difference between the arm waveguides has a 2π change, the output light is phase-modulated to zero phase and π, and functions as a phase binary phase modulator (PSK modulator) that has a constant signal light intensity at a signal timing. The output light also functions as a binary phase modulator when the upper arm and the lower arm in the LN of a Z-cut substrate are subjected to a push-pull driving for subjecting these arms to phase changes of the same amount and in opposite directions.
In the nest MZI modulator, binary phase modulation signal lights outputted from an Ich child MZI and a Qch child MZI are synthesized to have a 90° phase difference by having a quarter wavelength as a difference in the optical path length between the Ich-side and the Qch-side, thus providing QPSK signal light as shown in FIG. 2C modulated to have a quadri-phase. As described above, the nest MZI modulator can be allowed to operate as a QPSK modulator. Furthermore, Ich/Qch can have an arbitrary amplitude by setting the electric signal amplitude not only to a binary value but also to multiple values. Thus, a signal point at an arbitrary position on the electric field phase plane can be subjected to vector synthesis. Thus, this modulator is also called a vector modulator.
When the LN waveguide is subjected to a voltage for a long time, charge up for example causes a change in the refractive index of the waveguide, thus causing a phenomenon called a DC drift of interference condition shifts. A phenomenon called a temperature drift of a change in the refractive index is also caused depending on an environment temperature. The interference condition shifts as described above causes, in the child MZI, an error of a modulation operation point and causes, in the parent MZI, an error of an orthogonality in the relative phase of the Ich/Qch optical signal (i.e., error from the phase difference of 90°). These errors both cause a degradation of optical signal quality and are not preferred. Thus, the error amount must be sensed by an appropriate monitor means and must be compensated with adjustment.
The compensation of the error of the modulation operating point of the child MZI has been carried out, in the case of a modulator of an initial stage, by inserting, to a high-frequency input preceding stage, an electric circuit for synthesizing a high-frequency signal component called a bias tee and a DC bias component to superimpose a bias voltage on a modulation signal for compensation. However, the use of a bias tee causes a disadvantage where the low-frequency characteristic to an electric signal is substantially deteriorated. To prevent this, bias tees are not used in recent years and, as shown in FIG. 1A, exclusive bias electrodes BiasI and BiasQ for adjusting a modulation operating point to compensation/adjustment are frequently provided separate from a high-frequency electrode to apply a bias voltage.
In contrast with a high-frequency electrode, the bias electrode does not use a high frequency. Thus, the bias electrode does not use a distributed-constant design as in the traveling wave electrode and uses a simple lumped-constant design instead. However, the bias electrode applies an electric field to a waveguide that is basically in the same direction as that by the high-frequency electrode. Thus, the high-frequency electrode and the bias electrode have the same action from the viewpoint of direct current.
The 90° phase adjustment in the parent MZI is performed, as shown in FIG. 1A, by the bias electrode Bias90° provided in the parent MZI to adjust the relative phase of the Ich/Qch optical signal.
Next, the following section will describe a hybrid integrated-type nest MZI modulator obtained by combining a silica-based planar lightwave circuit (PLC) with an LN modulation array (see Non-patent Publication 2, FIG. 1 for example). This hybrid integrated nest MZI modulator is composed, as shown in FIG. 3A, of the connection of different waveguides of a PLC waveguide and an LN waveguide. The input-side three branch circuits and the output-side three confluence circuits are configured by a PLC waveguide. A modulation array section including a high-frequency electrode for converting an electric signal to a light modulation signal is configured by an LN waveguide.
Since the PLC waveguide has a very small EO effect, a single PLC waveguide cannot constitute a modulator. However, the PLC waveguide is a guided-wave medium having a very small loss for which the propagation loss is equal to or lower than one-tenth of that of the LN waveguide. At the same time, the allowable bending radius of the curved waveguide is about 2 mm and has a high design frexibility. Thus, a passive circuit can be used to realize various light circuits having a small loss. On the other hand, the LN waveguide has a propagation loss and an allowable bending radius larger than those of the PLC waveguide and thus is not suitable to constitute a complicated light circuit. However, since the LN waveguide has a high EO effect as described above, the LN waveguide is superior as a high-speed modulation circuit.
Thus, in the case of a complicated modulator such as a nest MZI modulator in particular, as shown in FIG. 3A, a passive circuit part such as a branch/confluence circuit uses a PLC waveguide and only a modulation array section uses an LN waveguide so that the former and the later have a hybrid integrated configuration. Thus, advantages of both of the PLC waveguide and the LN waveguide can be obtained. Thus, such a modulator can be realized that has a smaller loss and a better characteristic than those of a monolithic-type nest MZI modulator using an LN waveguide. This advantage is more remarkable with the increase of the complexity of the configuration of the modulator. This advantage is further remarkable in the case of a polarization multiplexing QPSK modulator requiring not only a branch/confluence circuit but also a polarization beam combiner for example (see Non-patent Publication 3, FIG. 1 for example).
The hybrid integrated-type nest MZI modulator operates basically in the same manner as in the above-described LN waveguide monolithic-type modulator. However, since the relative phase adjusting section is provided on the PLC, the 90° phase adjustment is carried out by a thermooptical (TO) phase shifter. The TO phase shifter uses a thin film heater formed in the clad surface on the waveguide to locally control the waveguide temperature to thereby control, via the TO effect, the waveguide refractive index (i.e., the phase of the guided light). Since the TO phase shifter uses heat, the response speed is on the order of millisecond. However, the above-described respective drifts occur very slowly. Thus, this speed is a sufficient speed to perform the bias adjustment such as the 90° phase adjustment. Although the adjustment of the modulation operating point of the child MZI may be similarly performed by providing a TO phase shifter on a PLC, the configuration of FIG. 3A uses a bias tee as described above so that a high-frequency electrode also can function as a bias electrode.